1. Field of the Invention
The present invention relates to a display unit and a method of fabricating the same, and more particularly, it relates to a display unit having a display electrode formed on an insulator film and a method of fabricating the same.
2. Description of the Prior Art
A transmission liquid crystal display unit employing a polycrystalline silicon TFT is known in general. For example, Japanese Patent Laying-Open No. 8-152651 (1996) discloses such a transmission liquid crystal display unit. FIG. 10 is a sectional view showing a pixel part 150 of the conventional transmission liquid crystal display unit disclosed in the aforementioned gazette. The structure of the pixel part 150 in the conventional transmission liquid crystal display unit is now described with reference to FIG. 10.
In the pixel part 150 of the conventional transmission liquid crystal display unit, a liquid crystal layer 103 filled with liquid crystals is formed between opposed transparent insulating substrates 101 and 102. The transparent insulating substrate 101 is provided with a display electrode 104 of a liquid crystal cell. The transparent insulating substrate 102 is provided with a common electrode 105 of the liquid crystal cell. The display electrode 104 and the common electrode 105 are opposed to each other through the liquid crystal layer 103. An alignment layer 136a is provided between the liquid crystal layer 103 and the display electrode 105, while another alignment layer 136b is provided between the liquid crystal layer 103 and the common electrode 105.
A polycrystalline silicon film 106 defining an active layer of a TFT 141 is formed on the surface of the transparent insulating substrate 101 closer to the liquid crystal layer 103. A gate insulator film 107 is formed on the polycrystalline silicon film 106. A gate electrode 108 is formed on the gate insulator film 107. A drain region 109 and a source region 110 of an LDD structure are formed on the polycrystalline silicon film 106. The drain region 109 of the LDD structure is formed by a low concentration region 109a and a high concentration region 109b. The source region 110 of the LDD structure is formed by a low concentration region 110a and a high concentration region 110b. The drain region 109 and the source region 110 of the LDD structure and the gate electrode 108 form the TFT 141.
The transparent insulating substrate 101 is provided on a portion adjacent to the TFT 141 with an auxiliary capacitor CS formed through the same step as that for forming the TFT 141. A storage electrode 111 of the auxiliary capacitor CS is formed in the polycrystalline silicon film 106 and connected with the source region 110 of the TFT 141. A dielectric film 112 is formed on the storage electrode 111. A counter electrode 122 of the auxiliary capacitor CS is formed on the dielectric film 112. The dielectric film 112, located on an extension of the gate insulator film 107, is identical in structure to the gate insulator film 107 and formed through the same step as that for forming the gate insulator film 107. The counter electrode 122 is identical in structure to the gate electrode 108 and formed through the same step as that for forming the gate electrode 108. Side wall insulator films 113 are formed on the side walls of the counter electrode 122 and the gate electrode 108. Insulator films 114 are formed on the counter electrode 122 and the gate electrode 108.
An interlayer isolation film 115 is formed on the overall surfaces of the TFT 141 and the auxiliary capacitor CS. The high concentration region 110b forming the source region 110 is connected to a source electrode 119 through a contact hole 117 formed in the interlayer isolation film 115. The high concentration region 109b forming the drain region 109 is connected to a drain electrode 118 forming a drain wire through a contact hole 116. An insulator film 120, an SOG film 132 serving as a planarization film and another insulator film 131 are formed on the overall surface of the device including the interlayer isolation film 115, the drain electrode 118 and the source electrode 119. The SOG film 132 serving as the planarization film is held between the insulator films 120 and 131. The display electrode 104 is formed on the insulator film 131.
The display electrode 104 is connected with the source electrode 119 through a contact hole 121 formed in the insulator film 120, the SOG film 132 and the insulator film 131. The aforementioned SOG film 132 fills up steps formed on ends of the auxiliary capacitor CS thereby flattening the surface of the display electrode 104. An aluminum alloy is generally employed as the material for the drain electrode 118 and the source electrode 119. Further, an ITO (indium tin oxide) film is generally employed as the material for the display electrode 104. The display electrode 104, the drain electrode 118 and the source electrode 119 are generally formed by sputtering.
In the aforementioned structure, the SOG film 132 serving as the planarization film is provided for the following reason: If large steps are caused on the display electrode 104, liquid crystal molecules cannot be homogeneously oriented in portions of the liquid crystal layer 103 located on the steps. When the liquid crystal molecules are heterogeneously oriented in the liquid crystal layer 103, the display electrode 104 cannot control light transmission and light interception of the liquid crystal layer 103, leading to a regular light transmission state. In this case, the contrast is lowered on the step portions regularly in the light transmission state. In the step portions, further, the thickness of the display electrode 104 is so reduced that the resistance value of the display electrode 104 is increased or the display electrode 104 is disadvantageously disconnected. In order to flatten the surface of the display electrode 104, therefore, the SOG film 132 is provided between the display electrode 104 and the insulator film 131 as the planarization film.
The term “SOG (spin on glass) film 132” generically indicates a film mainly composed of a silicon dioxide formed from a solution prepared by dissolving a silicon compound in an organic solvent. Spin coating is employed for applying the SOG film 132. More specifically, the solution prepared by dissolving the silicon compound in the organic solvent is dripped on a substrate while rotating the substrate. Thus, a coating of the solution is formed thickly on concave portions of steps defined on the substrate due to wiring and thinly on convex, to relax the steps. Consequently, the surface of the coating of the solution is flattened. Then, heat treatment is performed for evaporating the organic solvent and progressing polymerization, thereby forming the SOG film 132 having a flat surface.
The SOG film 132 includes an inorganic SOG film containing no organic component in the silicon compound as expressed in the following general formula (1) and an organic SOG film containing an organic component in the silicon compound as expressed in the following general formula (2):[SiO2]n  (1)[RXSiOY]n  (2)where n, X and Y represent integers, and R represents an organic group such as an alkyl group or an aryl group.
The inorganic SOG film contains large quantities of moisture and hydroxyl groups, has high hygroscopicity, is fragile as compared with a silicon oxide film formed by CVD (chemical vapor deposition), and readily cracked in heat treatment when its thickness is in excess of 0.5 μm.
On the other hand, the organic SOG film has portions where bonds are closed with alkyl groups or aryl groups and is hence inhibited from cracking in heat treatment, and its thickness can be set to about 0.5 to 1 μm. When employing the organic SOG film, therefore, an interlayer isolation film having a large thickness can be obtained and large steps defined on the substrate can be sufficiently flattened. However, the organic SOG film also contains moisture and hydroxyl groups although the quantities thereof are small as compared with the inorganic SOG film, and has high hygroscopicity.
Thus, the SOG film 132 serving as the planarization film contains moisture and hydroxyl groups, and has high hygroscopicity. The SOG film 132 partially discharges the moisture and hydroxyl groups contained therein due to temperature change or pressure change.
A photosensitive resin insulator film or another coating resin insulator film (a polyimide resin film, an acrylic resin film, an epoxy resin film or the like) can also be employed as the planarization film.
However, the resin insulator film or the organic SOG film, having organic groups in its components, discharges organic gas such as methane due to temperature change or pressure change.
Moisture, hydroxyl groups and organic gas discharged from the planarization film deteriorate the alignment layer 136a and the liquid crystal layer 103 or form bubbles in the liquid crystal layer 103 to cause defective display.
In order to prevent such inconvenience, there is a method of forming a film having a properly transmitting neither hydroxyl groups nor gas and performing treatment for suppressing transmission on the film.
Japanese Patent Laying-Open 8-152651 disclosing the aforementioned conventional structure describes a technique of forming the insulator film 131 on the SOG film 132 by plasma CVD and thereafter performing treatment for improving (modifying) the property of suppressing transmission of moisture and gas on the insulator film 131. This gazette also describes that a silicon oxide film, a silicon nitride film or a silicon oxynitride film is employed as the insulator film 131 and the treatment for modification may be performed by one of the following two methods:
In the first method, ions are implanted into the surface of the insulator film 131 formed by a plasma TEOS film or a plasma oxide film. The implanted ions are prepared from silicon ions, inert gas ions, arsenic ions, phosphorus ions or the like. In the second method, treatment with hydrogen plasma is performed on the surface of the insulator film 131 formed by a plasma TEOS film or a plasma oxide film.
However, the aforementioned method of modifying the conventional liquid display unit has the following problems: When forming the display electrode 104 of ITO, an ITO film must be formed on the overall surface of the insulator film 131 to be thereafter patterned into a desired shape by etching. In this case, the surface of the insulator film 131 is removed or damaged due to the etching for forming the display electrode 104 although the insulator film 131 is modified, and the effect of modifying the surface of the insulator film 131 is disadvantageously lost as a result. Therefore, it is difficult to solve such inconvenience that moisture or the like contained in the SOG film 132 is transmitted through the insulator film 131 to deteriorate the alignment layer 136a and the liquid crystal layer 103 or forms bubbles in the liquid crystal layer 103 to cause defective display after formation of the display electrode 104.
Deterioration of the alignment layer 136a conceivably also results from decomposition of the ITO film forming the display electrode 104. More specifically, the ITO film forming the display electrode 104 is decomposed to form indium and oxygen. Such indium and oxygen conceivably adhere to the surface of the alignment layer 136a, to deteriorate the alignment layer 136a. FIG. 11 shows a contrast ratio at the time of performing an aging test on the conventional liquid crystal display unit employing an ITO film as the display electrode 104. As shown in FIG. 11, the alignment layer 136a is deteriorated with time to disadvantageously reduce the contrast in the prior art.